CO2 lasers typically deliver radiation having a wavelength between about 9 and 111 micrometers (μm), with 10.6 μm, for which gain is highest, being a common output wavelength. Wavelengths in this range are strongly absorbed by materials including ceramics, glasses, plastics, wood, paints, and paper. This makes a CO2 a preferred laser for material processing applications involving these materials. Such applications include marking and drilling.
Many of these material processing applications require pulsed CO2 lasers having relatively high peak pulse powers, for example, 1 kilowatt (KW) or greater at relatively low average powers, for example, 50 Watts (W) or less. High pulse repetition frequencies (PRF), for example, between about a 10 kilohertz (KHz) and 100 KHz, and good mode-quality are also required.
Sealed-off, radio-frequency (RF) excited, Q-switched, diffusion-cooled CO2 laser are preferred for such applications, particularly if a high PRF is obtainable. The high PRF is important because the average output power of a pulsed, diffusion cooled CO2 laser increases with increasing PRF. The peak power of the Q-switched pulses decreases with increasing PRF beyond about 20 KHz. The average power becomes about equal to the continuous wave (CW) power at a PRF of about 100 KHz.
For maximizing average output power a CO2 laser needs to operate at a higher PRF than would be required for maximizing average power in a solid-state laser. This is because the CO2 molecule has a much faster, for example, about two orders of magnitude faster relaxation time of the upper excited laser state than common solid-state gain-media such as Nd:YAG and Nd:YVO4. Usually, a minimum acceptable PRF for CO2 laser material processing applications on the above-referenced materials is around 20 KHz.
In commercially available CO2 lasers, the usual means for Q-switching is a cadmium telluride (CdTe) electro-optic (E-O) crystal switch. A Q-switched laser operated by a CdTe E-O crystal switch is described in U.S. Pat. No. 7,038,093 assigned to the assignee of the present invention and incorporated herein by reference. The cost of a CdTe E-O switch is relatively high and can represent between about 25% and 30% of the total cost of a Q-switched CO2 laser. The E-O CdTe crystals are expensive, difficult to grow, difficult to polish and there are few suppliers thereof worldwide. Fast high-voltage electronic circuitry is required to drive a CdTe E-O switch. This circuitry is expensive and difficult to design. Intercavity optical components, such as polarizers and polarization rotators, cooperative with a CdTe E-O crystal, are required to provide the Q-switching. Such components, and indeed the CdTe crystal, introduce significant optical losses within the laser resonant cavity. Such losses, coupled with other resonator losses characteristic of CO2 lasers, can reduce the average output power by 30% or more at 100 kHz PRF compared with the CW average output power.
In the early years of development of the laser, attempts were made to mechanically Q-switch both CO2 and solid-state lasers using a rotating mirror or a rotating prism as an end-mirror of the laser resonant cavity to periodically convert the laser resonant cavity from a high loss (misaligned) to a low loss (aligned) state. Short-duration flash-lamps were used to optically pump the solid-state lasers to obtain one Q-switched output pulse per flash-lamp pulse. The rotating mirrors were driven by small electric motors or gas turbines. In order to obtain a fast optical switching time, required for fast-rise-time Q-switched pulses, a correspondingly fast rotational speed was needed. The fast rotational speed required a high degree of mechanical balancing for the rotating mirrors which added appreciable cost to the Q-switch laser.
These early mechanical Q-switching techniques were, and are still not suitable for the above-discussed material processing applications. The PRF obtainable is at best in the several hundreds of pulses-per-second range instead of in the multi KHz range required. Although there has been an awareness of these early mechanical Q-switching experiments, mechanical Q-switching is not included in any commercially available gas or solid-state laser. There is a need for a method of mechanical Q-switching that can provide Q-switching of a CO2 laser at PRFs in the kilohertz range required for material processing applications.